Heterogeneous networks are an efficient network deployment solution for satisfying the ever-increasing demand of mobile broadband services. In a heterogeneous network, a low- or lower-power node (LPN), for example a picocell, microcell or femtocell base station (NodeB), is placed in a traffic hot spot within the coverage area of a high- or higher-power node, for example a macrocell base station, to better serve nearby mobile devices. Deploying a low power node in a traffic hot spot may significantly reduce the load in the macro or other higher-power cell covering the area.
The traffic uptake of an LPN however may be somewhat limited. This is due to the transmit power difference between a macro base station (BS) and an LPN, e.g. 40 W vs. 5 W. This is illustrated in FIG. 1. In FIG. 1, the network 2 is shown as comprising a low power node 4 (e.g. picocell or microcell base station) placed within the coverage area of a macrocell base station 6. Each of the LPN 4 and macrocell base station 6 are connected to a radio network controller (RNC) 8 (typically via an lub interface in UMTS), which in turn connects to a core network 10. As downlink signal strength or quality is used as a basis for triggering a hand over between base stations, the border 12 of the LPN cell is determined by the downlink (DL) signal strength or quality as measured by a mobile communications device 12 (also known as a user equipment—UE). In high speed packet access (HSPA) networks, the quality of the common pilot channel (CPICH) is measured. The border 12 occurs at the point where the downlink signal strength or quality from the LPN 4 is the same as the downlink signal strength or quality from the macrocell base station 6. As the LPN 4 has a much lower transmit power level compared to the macrocell base station 6, the cell border 12 is much closer to the LPN 4 than the macrocell base station 6. However from the uplink (UL) perspective, the base station transmit power difference is irrelevant and the UE 14 would be best served by the base station to which it has the lowest path loss. Thus, for the UL, the cell border should be somewhere near the equal-distance point between the two base stations 4, 6 since at the equal-distance points the path loss from the UE 14 to both base stations 4, 6 is approximately equal.
The region 16 between the UL ‘border’ and DL border 12 is often referred to as the imbalanced region 16. In the imbalanced region, the UL from the UE 14 would generally be better served by the low-power node 4 (as it is closer to the UE 14 than the macrocell base station 6), but the DL would be better served by the macrocell base station 6. However, as cell selection is determined on the basis of the DL signal quality, a UE 14 in the imbalanced region 16 will generally be served by the macrocell base station 6, meaning that the UE 14 cannot take advantage of the better UL to the LPN 4.
One option for mitigating the imbalance is to extend the range of the LPN 4 by introducing an offset in the process of cell selection/reselection such that a UE 14 in a portion of the imbalance region 16 is served by the LPN 4 rather than the macrocell base station 6. This is referred to as ‘range extension’. For example, in a UMTS 3G network, a cell individual offset (CIO) can be used to adjust the cell border for UEs that are in the CELL_DCH state. CIO may be signalled in-band to a UE and thus can be specified for each UE. A UE uses the CIO to bias its mobility measurements (i.e. measurements of the DL signals from the LPN 4 and/or macrocell base station 6). For example, a UE 14 can make use of a weaker cell (i.e. LPN 4 when the UE 14 is in the imbalanced region 16) by applying a large CIO to the measured DL signal quality from the LPN 4. The signal quality can be either the common pilot channel (CPICH) RSCP (received code power) or CPICH Ec/No (energy per chip over noise power spectral density ratio).
Cell selection during the UMTS CELL_FACH state (i.e. the UE has no assigned dedicated radio resource) may be based on CPICH RSCP or CPICH Ec/No measurements, and parameters labelled QOffset1sn (for CPICH RSCP measurements) and QOffset2sn (for CPICH Ec/No measurements) can be used to bias cell selection. There are other cell selection priority parameters that may be used to give a carrier or cell a higher absolute cell selection priority. Each cell broadcasts the QOffset1sn and QOffset2sn values for use by all UEs 14 in all its neighbouring cells, and the values of QOffset1sn and QOffset2 are coordinated among cells that share the same cell border in order for the cell border to be consistent regardless of the cell in which a UE 14 is located.
FIG. 2 illustrates an example of cell range extension (CRE) in the network of FIG. 1. The DL border 12 corresponds to the conventional situation where no offset is applied to the signal quality measurements. However, if for the cell managed by the LPN 4 an offset is specified, the cell border 12 between the LPN 4 and macrocell base station 6 for UEs being served by the LPN 4 will be moved further from the LPN 4 (indicated by cell border 18).
Thus, moving the cell border 12 to increase the traffic uptake of a low-power node 4 is an attractive enhancement for heterogeneous network deployments. It is beneficial from an UL perspective since the UE 14 will be served by the base station to which the path loss is lower (i.e. the LPN 4 in FIGS. 1 and 2). However, care must be taken not to extend the range too far. Excessive range expansion leads to degradation in the DL performance for a UE 14 served by a low-power node 4 since the received DL power from the serving low-power node 4 in the imbalanced region 16 is weaker than that from the non-serving macrocell base station 6. In addition to the desired signal being weaker, the interference from the macrocell DL is also stronger. Poor DL performance may also impact UL performance since the UL data channel (for example, enhanced-dedicated channel—E-DCH) requires reliable DL signalling (e.g. enhanced-absolute grant channel—E-AGCH, enhanced-relative grant channel—E-RGCH and enhanced-DCH hybrid ARQ indicator channel—E-HIGH). Poor DL performance may also cause handoff problems when the signalling radio bearer (SRB) is carried by the downlink shared channel (HS-DSCH).
To avoid such asymmetric behaviour, some form of UL/DL separation is desirable. Inherently in an HSPA system, this occurs to a certain degree by virtue of uplink soft handover (SHO) that may be configured when the UE is in CELL_DCH state. In uplink soft handover, a UE 14 can be connected to two or more cells, and each cell receives the UL transmissions from the UE 14 and passes them to the RNC 8 which selects the signal with the highest quality. A form of soft handover (referred to herein as extended soft handover) may be employed to leverage the SHO feature to an even greater degree. With extended soft handover, a UE 14 in the imbalanced region 16 continues to be served by the macrocell base station 6. However, in the uplink the UE establishes a connection to both the macrocell base station 6 and LPN 4. This is illustrated in FIG. 1, in which the UL connection from the UE 14 in the imbalanced region 16 to the serving macrocell base station 6 is shown by arrow 20 and the UL connection to the LPN 4 is shown by dashed line 22. In the (extended) SHO region (i.e. the CRE region) the path loss to the LPN 4 is lower than to the macrocell base station 6, hence the power control loop is effectively “steered” by the LPN 4. In this way, a partial UL/DL separation is achieved in that the uplink transmissions are most often decoded correctly at the LPN 4 (due to the lower path loss) while the downlink transmissions still occur from the macrocell base station 6 (which has the highest received quality at the UE 14).
A shortcoming of the (extended) soft handover approach is that it is available only for UEs in CELL_DCH connection state. For UEs 14 operating in the CELL_FACH state (i.e. in which there is no dedicated physical channel allocated to the UE 14), the current standard does not support soft handover in order to maintain the simplicity and reduced overhead of this connection state. Without CRE, a UE 14 in the imbalanced region 16 in CELL_FACH state always connects to the macrocell base station 6, thereby compromising the quality of the uplink. Consideration of the CELL_FACH state is important since this state is primarily useful for short data transmissions which commonly occur with smart phone traffic. It has been found that this type of traffic is already predominant in networks, and is expected to grow further.